1. Field of the Invention
The present invention generally relates to a DAC (digital/analog converter) with adjusting digital codes corresponded to reference voltages, and more particularly, to a system in which a novel DAC designed with an associated circuit is used in non-linear digital-code-to-voltage transfer applications.
2. Description of the Prior Art
Recently, the technique in the field of displays has grown rapidly with the fast development in opto-electronics. However, for a display (a television having a conventional picture tube, a state-of-the-art thin film transistor-liquid crystal display or a plasma display panel), the realization of the correction of Gamma parameters has become a major factor of the image quality.
Gamma correction is achieved by the digital-code-to-voltage transfer function of the internal DAC of a data driver. However, the digital-code-to-voltage transfer function for Gamma correction is influenced not only by the opto-electronic characteristics but also by the coding of the image and the users"" settings so that it may exhibit different destination curves. Please refer to FIG. 1, which is a graph illustrating the variation of the digital-code-to-voltage transfer curve required for the data driver of the display, in which the digital-code-to-voltage transfer curve moves with respect to the adjustable Gamma reference voltages. As shown in the drawing, as the minimum driving voltage varies, different digital-code-to-voltage transfer curves can be obtained. For example, if it is given a driving voltage range of 1xcx9c5V, the obtained digital-code-to-voltage transfer curve is different from the digital-code-to-voltage transfer curve obtained when it is given a driving voltage range of 1xcx9c5V.
The internal DAC of a state-of-the-art data driver generates different digital-code-to-voltage transfer curves by changing the reference voltage values. The voltage dividing circuit used in the prior art is shown in FIG. 2, in which the voltage dividing circuit 1 is composed of a plurality of voltage dividers 5 and the output voltages are constant. The typical digital-code-to-voltage transfer curve is as shown in FIGS. 3A and 3B. In general, the corresponding digital codes are not adjustable, and the different digital-code-to-voltage transfer functions can only be implemented by changing the reference voltages. Taking FIG. 3B for example, the Gamma reference voltages range (V1xcx9cV5) are adjustable with the corresponding digital codes (0, 32, 192, 230, and 255) fixed.
However, such a conventional technique, as shown in FIGS. 2 and 3, has one major setback: small degree of freedom in implementing the digital-code-to-voltage transfer function. The digital codes with respect to the external Gamma reference voltages of the data driver used in the mentioned conventional techniques are fixed. This reduces the degree of freedom for the realization of the correction of Gamma parameters and leads to greater difference of the transfer curve and the destination function. As a result, the image quality of the display is adversely affected.
It is mainly due to the fact that the relation between the digital codes and the signal voltages as well as the relation between the digital code and the luminance of a display are non-linear, and such non-linearity must be corrected by using Gamma parameters, so that the signal voltages and the digital codes or the luminance can meet the requirement of linearity and thus high image quality can be obtained. Accordingly, to overcome the non-linear characteristics in the prior art, the present invention provides a novel DAC circuit in which the digital signals are converted into analog signals before being delivered into the display, so as to improve the image quality.
In order to overcome the problem in the prior art, it is thus the primary object of the present invention to provide a DAC (digital/analog converter) with adjustable digital codes corresponding to reference voltages, in which a novel DAC designed with an associated circuit is used in non-linear digital-code-to-voltage transfer applications, so as to provide the display with analog voltage signals. In such a DAC, the Gamma reference voltages as well as the corresponding digital codes are adjustable. It provides a greater degree of freedom for the realization of the correction of Gamma parameters so as to fit the curve representing the transfer function of the destination digital codes and the voltages required by the data driver and to precisely implement the digital-code-to-voltage transfer function.
The present invention also provides a voltage dividing circuit not only applicable to the Gamma correction system but also suitable for use in other non-linear transfer applications.
In order to achieve the foregoing object, the present invention provides a DAC with adjustable digital codes corresponding to reference voltages, comprises: a plurality of programmable switches, receiving different reference voltages respectively and operating under the control of a switch control unit; a switch controller, connecting the output terminals of the plurality of programmable switches and voltage dividers with corresponding digital codes according to the corresponding digital code signals from a signal source, so as to apply the reference voltages to the digital codes; a digital/analog voltage dividing circuit, composed of a plurality of voltage dividers serially connected, wherein the input terminals of the digital/analog voltage dividing circuit are connected to the plurality of programmable switches, so as to output a plurality of voltage values of the digital codes with respect to the reference voltages according to the switch controller; and an output voltage selecting unit, connected to the output terminals of the digital/analog voltage dividing circuit, so as to select a series of voltage dividing points and deliver a set of voltage values to the output terminal when the number of the voltage dividing points is larger than the number of the output voltages.
It is preferable that the voltage dividers are composed of capacitors, resistors, or a combination of both capacitors and resistors.
It is preferable that each of said plurality of output voltage selecting unit selects and outputs a set of proper divided voltages when the number of divided voltages is larger than the actual number of the output voltages. Wherein each gamma reference voltage except the first and last ones is connected to two programmable switches to select the two corresponding voltage dividing points according to said switch controller. And then each of said plurality output voltage selecting unit can select a set of proper voltage outputs and can skip some voltage dividing points as so to meet the requirement that the number of divided voltages is larger than the actual number of the output voltages.
It is preferable that the first voltage and the last voltage are connected to only one individual switch, so as to select the corresponding voltage dividing points with respect to the reference voltages.
It is preferable that the corresponding digital codes with respect to the mth and the (m+1)th reference voltages are adjacent, non-adjacent, or alternating.
It is preferable that the output voltage selecting unit is implemented by a front-end output voltage selecting unit and a back-end output voltage selecting unit.